Computed tomography systems typically include an x-ray source which emits a fan beam directed through an object to be imaged and received by an x-ray detector array. The x-ray source and detector array are orientated to lie within an x-y plane of a Cartesian coordinate system, generally referred to as the "imaging plane". The x-ray source and detector array may be rotated together on a gantry within the imaging plane and around the image object, i.e., around the z-axis of the Cartesian coordinate system. Rotation of the gantry changes the angle at which the fan beam intersects the imaged object, and such an angle is generally referred to as the "gantry" angle.
The detector array has a plurality of detector elements, and each detector element generates a signal indicative of the intensity of transmitted radiation received by the detector element. At each gantry angle, such detector element signals are collected, digitized and saved. Such data sometimes is referred to as projection data. Intensity signals from each of the detector elements for a particular gantry angle may, for example, be stored in a projection data array. The gantry is rotated to a number of gantry angles and projection data is collected at each such gantry angle to form a tomographic projection set.
Each acquired tomographic projection set may be stored for later processing to reconstruct a cross sectional image, or slice, according to algorithms known in the art. The reconstructed image may be displayed on a conventional CRT tube or may be converted to a film record by means of a computer controlled camera.
The x-ray source is ordinarily an x-ray "tube" which includes an evacuated glass x-ray envelope containing an anode and a cathode. X-rays are produced when electrons from the cathode are accelerated against a focal spot on the anode by applying a high voltage across the anode and cathode. The voltage applied across the anode and cathode, the current flowing between the anode and cathode, and the duration of the exposure, for a given x-ray procedure, is generally referred to as the "exposure technique".
To increase the amount of projection data which may be collected during a scan, it is known to utilize two adjacent rows of detector elements. Specifically, the detector elements may be positioned side-by-side in two adjacent rows, and the x-ray source may be aligned with the detector array so that the output x-ray may be substantially simultaneously received by detector elements in both rows. A collimator typically is positioned at the interface between two detectors elements in respective rows. Such an apparatus sometimes is referred to as a "twin beam scanner".
The detector array may have ionization type detector elements or solid state detector elements as are known in the art. Both detector element types exhibit changes in their sensitivity to x-rays as a function of the position of the fan beam along their surface. Such changes in signal strength during the acquisition of a tomographic projection set may produce undesirable ring like image artifacts in the resultant reconstructed image.
For example, the sensitivity response of known detector elements at the detector element edge region is low. As a result, the detector element may not generate a highly accurate intensity signal for x-rays which are received at the detector element edge region.
Although the post-patient collimators in the twin beam scanner described above may block the x-rays from the adjacent edges of adjacent detector elements in respective rows, the x-rays may still be received within such detector element edge regions. In addition, use of collimators and detector elements arranged in respective adjacent detector rows may reduce the dose efficiency of the system since at least the projection data represented in the attenuated x-ray beam portions which are blocked by the collimators is lost. Further, due to the detector characteristics at the detector edge region, the projection data represented in the attenuated x-ray received at the detector edge region does not facilitate reconstructing an image of high quality. Such non-uniformity in the detector response at the edges typically results in the generation of artifacts when a sloped object is scanned.
In addition to the projection data discontinuities, calibrating such a twin beam scanner also is difficult. Particularly, although the x-ray source may be initially correctly aligned so that an output x-ray is centered to be received in substantially equal portions by adjacent detector cells, an x-ray may become off-centered during a scan operation. For example, as the x-ray source heats up, the thermal expansion of the anode may cause the focal spot to move. Also, as the gantry rotates, mechanical stresses on the gantry and x-ray source may cause additional focal spot motion. Such movement of the x-ray focal spot is particularly troublesome with the twin beam scanner described above since such movement may result in an increase in the percentage of an x-ray received in the edge region of a detector element. If such an increase occurs during a scan, the initial calibration for the detector element may be incorrect, and such a condition may result in additional artifacts being generated.
Known twin beam systems may be utilized to perform a helical scan. To perform a helical scan, the object to be imaged is moved along the z-axis while the gantry rotates. In a twin beam system, double interwoven helixes are mapped by the projection data. In a helical scan, if a helical pitch of about or equal to 1:1 is selected, a significant amount of "overlap" may occur. Specifically, a 1:1 helical pitch means that the table increment in one gantry rotation equals the width of one slice. The projection data collected by one detector element in the first row may substantially overlap, or be substantially identical to, the projection data collected by an adjacent detector element in the second row in the next gantry rotation. Such overlap is undesirable because the duplicate projection data does not significantly facilitate generating images for multiple slices.
Accordingly, it would be desirable and advantageous to provide twin beam scanner which facilitates collecting projection data within a detector element region which provides a more uniform sensitivity response than the detector edge region and which substantially eliminates projection data overlap in a helical scan. It would also be desirable and advantageous to provide such a scanner which does not generate projection data discontinuities and may be more easily calibrated.